The field of the invention pertains to the ventilation of attics and roof undersides and, in particular, to roofs of wood frame construction having fiberglass bats inserted between the rafters of pitched roofs or the joists of substantially flat roofs.
In typical wood frame roof construction where the interior ceiling is attached to the rafters or joists and substantially follows the roof pitch, the insulation is placed between the rafters or joists. Thus, the insulation lies between the roof sheathing and the ceiling.
One of the most common, if not the most common insulation for wood frame roofs as well as steel frame roofs is fiberglass bats manufactured by such well known companies as Owens-Corning, Inc., Manville Corporation and Certainteed Corporation. The fiberglass fibers are very loose but randomly intermingled so as not to fall apart. A layer of paper or foil is attached to one or both sides of the fiberglass bat.
Before installation the fiberglass bats are tightly rolled or otherwise compressed to reduce bulk in shipment. Upon installation the bats greatly expand, normally filling the entire volume between the rafters or joists and between the ceiling and roof sheathing. As a result the fiberglass expands against the underside of the roof sheathing effectively preventing the movement of an air layer along the underside of the roof sheathing.
The prevention of air circulation along the underside of the roof sheathing has several detrimental effects. Under direct sunlight the roof sheathing temperature increases to a substantially higher temperature than otherwise thereby reducing the effectiveness of the fiberglass insulation.
Any moisture that collects under the roof sheathing in cold weather is retained in the fiberglass thereby reducing the insulating effectiveness of the fiberglass and also causing deterioration of the roof sheathing. Proper circulation of air beneath the roof sheathing can reduce the temperature extremes of the roof and substantially prevent the collection of moisture under the roof sheathing.